The present invention relates to an optical isolator and an optical part having a heat-resistant anti-reflection coating. More particularly, the invention relates, in a first aspect of the invention, to an optical isolator provided with a polarizer and analyzer of polarizing glass to utilize the Faraday effect. In another aspect of the invention, the present invention relates to an optical part provided with a heat-resistant anti-reflection coating and suitable for use at an elevated temperature of 250.degree. C. or higher.
An optical isolator is constructed from a polarizer and analyzer as well as a Faraday rotator and permanent magnet installed therebetween and these parts are built in a holder. While several particular modifications are known for an optical isolator, it is usual to use polarizing glass for the polarizer and analyzer in a compact optical isolator in view of the possibility of accomplishing a high quenching ratio and a small-thickness design. Several commercial products of polarizing glass are available on the market including Polacore (a product by Corning Glass Works Corp.).
Japanese Utility Model Kokai 5-96830 discloses a structure of an optical isolator, as is illustrated in FIG. 7 of the accompanying drawing, consisting of a polarizer 1, analyzer 7 and Faraday rotator 8 built in a tubular holder 6 and a tubular magnet 9 in which a holder ring 2 is used in order to facilitate exact position adjustment of the polarizer 1, analyzer 7 and Faraday rotator 8. A diaphragm 10 is provided to limit the light incident on the polarizer 1. It is a known method to use an adhesive of an organic polymeric resin for bonding and fixing the polarizer 1 and analyzer 7 to the respective holder rings 2 and the Faraday rotator 8 to the tubular magnet 9.
When the polarizer 1, analyzer 7 and Faraday rotator 8 are bonded to the holder rings 2 and to the tubular magnet 9 by using an adhesive, however, the adhesive has a relatively large thermal expansion coefficient so that a great displacement from the exact setting of the inclination angle is sometimes caused by the variations in the ambient temperature. This phenomenon unavoidably results in a deviation of the optical axis once optimized by adjustment due to the variation in the ambient temperature. In addition, volatile organic matters are emitted from the adhesive to cause contamination and degradation of the laser as the light source decreasing the long-term reliability of the optical isolator.
As a solution for the above mentioned problem of low longterm reliability of the optical isolator, it is known that the polarizer, analyzer and Faraday rotator are bonded and fixed to the respective holding parts by the use of a solder or brazing alloy or low melting-point glass. As the solder alloy for this purpose, gold-based solder alloys free from lead are used in recent years in consideration of the problem of environmental pollution due to heavy metals such as lead.
In bonding and fixing the polarizer, analyzer and Faraday rotator to the holding parts by soldering, one of the very important requirements is that the difference in the thermal expansion coefficients should be as small as possible between the holding parts and each of the polarizer, analyzer and Faraday rotator because, if the difference in the thermal expansion coefficients is great, cracks are possibly formed in the polarizer, analyzer and Faraday rotator due to the thermal stress resulting in complete loss of the performance of the optical isolator. This phenomenon is particularly remarkable in the bonding and fixing works between a holder ring and a polarizer and analyzer made from polarizing glass.
Following methods are known to solve the problem due to crack formation mentioned above.
(1) A method is disclosed in Japanese Patent Kokai 5-11215 in which the optical element and the holding part are bonded with intervention of a spacer having an intermediate thermal expansion coefficient. The material of the spacer is preferably a ceramic and the spacer has a configuration of a circular ring, optionally, provided with a slit or cutout, a configuration to match the metallized part for soldering in the optical element or a square configuration with a circular openwork at the center, optionally, divided into portions. PA1 (2) Japanese Utility Model Kokai 6-4735 discloses a method in which the holding part is provided with a slit. PA1 (3) Japanese Patent Kokai 6-167675 teaches forming the holding part from an alloy having a thermal expansion coefficient close to that of the polarizing glass such as Fe-32Ni alloy and Fe-42Ni alloy. PA1 (4) Japanese Patent Kokai 6-34861 teaches a method in which the side surfaces of the optical element are metallized and the side surface of the optical element and the holding part are bonded with intervention of the metallized layer. PA1 (5) Japanese Patent Kokai 6-67119 discloses a method in which the surfaces of the optical element, excepting the surfaces to serve for the light beam transmission, are provided with a metallized layer patterned in at least four concentrical rings with regular spaces between the adjacent rings, of which the overall metallized area is in the range from 5 to 25% based on the area for the incident light into the optical element.
The inventors have tried to test the practicability of the above described first to third methods for bonding and fixing of a polarizer and analyzer of polarizing glass to a holder ring by soldering and found a problem that, though without formation of cracks, the quenching ratio is not sufficiently high in the polarizer and analyzer after bonding so that the optical isolator using the same cannot work at a high quenching ratio.
The reason therefor is, like the reason for the above mentioned crack formation, presumably the thermal stress due to the difference in the thermal expansion coefficients between the polarizer and/or the analyzer and the holder rings. Although cracks are not formed in the above mentioned first to third methods by virtue of the decrease in the thermal stress in the polarizer and analyzer, namely, relaxation of the thermal stress is still incomplete not to ensure a high quenching ratio as an optical isolator.
Following are further known methods for decreasing the thermal stress to accomplish a high quenching ratio of the polarizer and analyzer.
The inventors have tried to test the practicability of the above described fourth and fifth methods for bonding and fixing of a polarizer and analyzer to holder rings by soldering and found a problem that the distribution of the quenching ratio is not uniform within the plane as is illustrated in FIG. 6 and the quenching ratio decreases from the center toward the bonded parts. This result leads to a consequence that optical isolators cannot be so compact as to be smaller than a certain limit and accordingly the costs thereof cannot be decreased as desired. The reason therefor is as follows.
It would be a possible way for obtaining an optical isolator of a high quenching ratio by using a polarizer and analyzer bonded to the holder rings by the above mentioned fourth or fifth method to use a polarizer and analyzer having a sufficiently larger surface area than the light-transmitting surfaces. Namely, the optical isolator can exhibit desirable performance by utilizing the portions of a high quenching ratio only fully isolated from the bonding parts. This method, however, has a problem that the compactness of the optical isolator has a lower limit and the optical isolator cannot be more compact than the limit. In view of the expensiveness of available commercial products of polarizing glass in general such as Polacore mentioned above, on the other hand, it is an important requirement that the area of the polarizer and analyzer should be as small as possible in order to minimize the costs of optical isolators. Accordingly, it is almost impossible by the conventional technology to have compatibility between these requirements for compactness and inexpensiveness of optical isolators with a high quenching ratio.
The above described attempts undertaken by the inventors have led to a conclusion that a quenching ratio sufficiently high for an optical isolator could hardly be accomplished by the first to third methods only teaching selection of the materials and configurations of the holder rings although the problem of crack formation could be solved thereby. Further, a compact and inexpensive optical isolator having a high quenching ratio can hardly be obtained by utilizing the above described first to fifth methods either singly or in combination because of the great non-uniformity in the distribution of the quenching ratio within the plane although the quenching ratio can be partially high enough.
While the overall stress by bonding can be reduced by decreasing the bonding area, the disclosure in Japanese Patent Kokai 6-67119 teaches that the bonding area cannot be decreased below a certain level in order to ensure a bonding strength suitable for practical use of the optical isolator. Thus, it is a conclusion that none of or no combinations of the above described first to fifth methods could satisfy all of the requirements for an optical isolator relative to a high quenching ratio, high bonding strength, compactness and inexpensiveness simultaneously.
Turning now to the second aspect of the invention, it is usual that various kinds of optical parts or elements such as optical glass bodies, single crystals, plastic-made optical parts and the like are provided with an anti-reflection coating film on the surface with an object to decrease reflection of light. Such an anti-reflection coating film is either a monolayered film or a multilayered film depending on the particular intended application of the optical part. When the anti-reflection coating film is a monolayered film, the film is formed from TiO.sub.2, Ta.sub.2 O.sub.5, ZrO.sub.2 and the like to have a very small thickness while the coating film having a multilayered structure consists, in many cases, of the above mentioned thin film of a titanium oxide and the like which is overlaid with another thin film having a low refractive index such as SiO.sub.2. These anti-reflection coating films are formed on the surface of the optical part by the method of vacuum vapor-phase deposition, ion plating, sputtering or the like.
The reflectivity of an anti-reflection coating film depends on the wavelength of the incident light, angle of incidence of the light, optical film thickness, i.e. the product of the refractive index and the thickness of the film, of each layer and structure of lamination of layers. When a high refractive index is desired, the anti-reflection coating film is formed from an oxide of titanium such as TiO.sub.2 while titanium oxide used for forming the anti-reflection coating film on an optical part include TiO, TiO.sub.2, Ti.sub.3 O.sub.5 and the like or, in a general formula, TiO.sub.x of which the value of the subscript x largely depends on the method of film formation, conditions of film formation and types of the source material for film formation. As a method for the formation of a coating film of TiO.sub.x, K. Narasimha Rao, et al. in Journal of Vacuum Science and Technology, A11, pages 394-397 (1993) teach a method of vapor-phase deposition in which a TiO.sub.2 film is formed by the deposition of TiO onto the substrate surface at 250.degree. C. while the substrate surface is under irradiation with oxygen ion beams. H. Demiryon, et al. in Journal of Vacuum Science and Technology, A2, pages 1457-1460 (1984) teach a method in which TiO.sub.2 is deposited onto the substrate surface under irradiation of the substrate surface with ion beams of an argon-oxygen mixture.
On the other hand, it is sometimes the case that an optical part is bonded and fixed to another optical part or to a casing by using a metallic or inorganic bonding agent such as solder alloys and glassy materials of low melting point. When these bonding agents are used for such a purpose, needless to say, the bonding agent must be heated up to or above the melting point of the solder alloy or softening point of the glass. When the solder alloy is a gold-tin alloy, for example, the heating temperature must usually be 280.degree. C. or higher.
A problem caused by this heating for the bonding work of optical parts is that the anti-reflection coating film suffers a change in the optical film thickness. When an optical part provided with an anti-reflection coating of TiO.sub.2 is bonded and fixed with soldering under heating up to 280.degree. C. or higher, for example, an increase is caused in the reflectivity of the anti-reflection coating film after bonding. Incidentally, the above mentioned literature by K. Narasimha Rao, et al. does not assume a case in which the heating temperature of a TiO.sub.2 film in the bonding works exceeds 250.degree. C.